Effects of Hydrogen on the Engineering Properties of MONEL Nickel-Copper Alloy K-500

A laboratory investigation was conducted to determine the effects of cathodically charged hydrogen on the mechanical properties of MONEL⁽¹⁾ nickel-copper alloy K-500, and to investigate the relationship between material hardness and susceptibility to hydrogen embrittlement. Tensile specimens having hardnesses of 26 to 41 Rockwell C were made the cathode in a solution of 4% sulfuric acid saturated with carbon disulfide. Charging times of up to 16 days were used, after which the specimens were immediately analyzed for hydrogen and pulled to failure. Hydrogen absorption in alloy K-500 is characterized by a relatively high solubility compared to other face-centered-cubic alloys (austenitic stainless steels, for example), but the alloy exhibits the low diffusion rates associated with the face-centered-cubic matrix. Hydrogen charging of the alloy then results in material having a brittle surface layer and a ductile core. Cracks form in the brittle surface at low strain values. The cracks act as notches and decrease the ductility of the unembrittled core. The amount of hydrogen absorbed by the alloy does not depend on the hardness of the material. Harder material, however, shows greater ductility loss because it is more notch sensitive.